Matrix for making light transmitting screens



July 23, 1946. o. c. MARTIN MATRX FOR MAKING LIGHT TRANSMITTING SCREENS2 Sheets-Sheet 1 Original Filed June 28, 1939 0275 6. MRW/v lNvEN-roaArme/v w July 23, 1946. 0. c. MARTIN 2,404,448

MATRIX FOR MAKING LIGHT TRANSMITTING SCREENS Original Filed June 28,1939 2 Sheets-Sheet 2 ,Fra 5.

Or/s 6. MMU/xg INVENTOR TOPE.

Patented July 23, 1946 MATRIX FOR MAKING'LIGHT TRANSMITTING SCREENS Y-Otis C. .Martin, Los Angeles, Calif.; Marion 5E. Martin administratrixofsaid Otis C. Martin,

deceased AOriginal `application June 28, 1939, Serial No. 281,625.Divided and this application VMarch 16, 1942, SerialvNo. 434,810

2 Claims. 1

My invention relates to a matrix for making projection screens whichfinds particular utility when employed for ref-photographing motionpictures.

This application is a division of my copending application Serial No.281,625, filed June 28, 1939, now Patent No. 2,315,721, and entitledMatrix element for making light transmitting screens,

One of the most common uses of light transmitting screens forrephotographyof motion pictures is in connection with the process knownto the motion picture industry as back projection. Back projectionconsists in projecting a, motion picture of a landscape, street scene,or other background material upon a translucent projection screen.Actors participating in the enactment of a motion picture being filmedare positioned before this screen and the actors and the screen arephotographed simultaneously so that the completed photograph portraysthe actors in the environment Whichvis back projected. on thetranslucent screen.

Considerable difficulty has been encountered in the use of this processas regards the obtaining of the Arequired uniformity of illuminationthroughout theentire surface of the translucent screen when viewedfromlthe location of the camera lens. The'use of a cloth screen for thisprocessis unsatisfactory in that the cloth absorbs too much light kandprevents adequate exposure of the film used in the rephotographingcamera. As a result, ground glass has been usedin the past but has notbeen satisfactory due vto wthe fact that the ground glass, While tendingto cliffusethe light transmitted through it, tends nevertheless t0transmit the greater proportion of the light in Ythe .same generaldirection as was followed by the light striking the screen. The screen,therefore, when viewed from .the location of the rcamera lens displayedla very bright central area, becoming increasingly darker 'as the edgeof the screen was approached.

The use of Lucite(atransparentplasticanaterial `having a vhigh lighttransmitting -efliciency and-afrelatively highinde'x of-re'fraction)Ihas resulted in the manufacture lof extremely satis- -factory. screens:in the-smaller I'sizes, f not exceeding six Ifeet '1in Veither Widthorheight. These `'smal-1er screensusually-either incorporatea great numberof individual lens elements -for redirectingthe'lightasitis'passe'd'through the transparent'screen'or are made in the 'form of-avplane y'Fresnel -lens,'the 'individual prismatic faces of lWhich 'areformed 'by' cuttinga continuous 4scroll 2 or spiral .groove oftherequisite'cross section in the .front 'face of'a sheet of Lucite Screensemployed for the back projection process oftentimes are required tohaveasize as large-as 30 x 40 `feet and until the presentinvention it hasbeen impossible to construct a screen of this size of Lucite whichemploys the desired optical principles which are incorporated in thesmaller sized screens.

It is an object of my invention to .provide a novel matrix upon whichliquid Lucite may be cast to. provide alight transmitting screen of anydesired size having iformed integrally-therewith va meansfor'redirectingthelight which is passed therethrough.

v.It is also an object of my invention to provide amatrix of thecharacterset forth in the precedingparagraph which-includes means fordening a continuousspiral groove of any selected cross section in the.Lucite which may be cast thereupon.

It .is anadditional object of my invention to provide afmatrixiof thecharacter'set forth in the preceding paragraph .which comprises a `sheetformed by winding a nat interlocking ribbon spirally about itself.

Other obiectsland advantages of my invention will'be apparentAfroma'study ofthe following specifications, .read :in Aconnection with theaccompanying drawings, wherein:

Fig. '1 is an `elevational View illustrating the appearance of a lightvtransmitting yscreen constructed-bythe matrix fof my invention;

Fig. A.2 is an enlarged vfragmentary cross ysectional lview takensubstantially .along the line II-II of`Fig. 1 illustrating in detail themechanism -by which the screen may be mounted in its frame andillustrating one type of spiral groove which may be provided inthescreen face; Fig. 3 is a plan viewof a matrix core member upon whichya 'suitable ribbon material may be `wound to providea spiral matrix;

Fig. Vi is lanexplo'ded Vperspective view illustrating the! manner inwhichthe endtofthe ribbon material-may be securedtoithe core memberwhich isi1lustratedinFig3; and

Fig. 5 is a fragmentary cross section asimilar toFig. T2 butillustrating thefmanner in which the adjacent turns of ribbon .materialeinterlo'ck 'with eachother to providera Vmatrix upon which a liquidfLuoit`el"maylbefcast to yprovide a'screen of the desired character.

Referring -to the drawings, SI have illustrated inFig. lfoneformofscreen whichmay'beeonstructed according'to myinvention. The screen meanscooperating with suitable lacings 3 to support the screen structureWithin any suitable supporting frame 4. The frame 1l is adapted to holdthe screen in a substantially vertical plane and for this purpose may besuspended from suitable overhead supporting structure or may rest upon asupporting surface such as a floor 5 by means 0f feet or legs 6.

It has been found that if the screen l is made of a transparent yplasticof the type known in the trade as Lucite and is provided on its frontface with a series of concentric or spiral grooves l of the proper crosssection to provide a step cut screen surface having a plurality of lightrefracting facets, light which is projected upon the back of the screenby a motion picture projector will be redirected by such facets into theobjective of a camera disposed before the screen.

The cross section of the grooves l may be so chosen as to redirectsubstantially all of the light which falls upon the rear face of thescreen, thus, in effect, forming a Fresnel type of lens.

It has been found, however, that if the grooves l are so constructed,the motionpicture projector which is disposed behind the screen and thecamera which is disposed in front of the screen must be very carefullyaligned with the axis of the lens formed by the screen and the distancebetween the screen and the projector and between the screen and thecamera must be verycarefully adjusted in terms of the effective focallength of the screen. For this reason it is preferred to make thegrooves l of a.

screen surface so as to be directed generally toi wards the objective ofthe projector which is disposed behind the screen. The other face 9 ofthe groove l is preferably made curved in cross section, the lowermostpart which defines the bottom of the groove 'l being disposedsubstantially parallel to the front face of the screen I and the partwhich intersects the front surface ofthe screen being disposed at aconsiderable angle thereto.

The angle at which the surface 9 intersects the front face of the screenis controlled by the camera and projector angle which is subtended bythe screen, it being found that an angle of substantially 60 at theintersection of the face 9 with the front surface of the screen issuflicient to accommodate the total camera angle ofy approximately 40.

It will be observed that with this construction, the light which isprojected upon the rear surface of the Screen by the motion pictureprojector will strike the screen at an angle somewhat in the fashionindicated by arrows Ill and li in Fig. 2. It will be observed that thedirection of these arrows is substantially parallel to the direction ofthe plane face 8 so that substantially the entire screen surface whichintersects the rays HB and Il is comprised of a Imultiplicity of lightrefracting facets comprising the curved surfaces 9. The rays IG and Ilstrike the curved surfaces 9 at an acute angle and are consequentlyrefracted thereby, the amount of refraction depending upon the angle atwhich the rays lil and l l strike this surface.

It will be observed that the greater part of the light will strike thiscurved surface at such an angle as to cause it to be refracted in theproper direction to enter the lens of a camera disposed before thescreen irrespective of whether the particular groove in question islocated at the center of the screen or at the extreme edge thereof.

The screen, when lighted from behind and viewed from the position of themotion picture camera lens, appears to have a bright line of light woundin spiral fashion in the same man ner as are the grooves l. In orderthat the screen will appear as though uniformly lighted, I prefer tomake the spacing between each of the grooves l extremely small ascompared with the dimensions of the screen. It has been found that acenter to center distance between each of the adjacent grooves 'l ofless than le of an inch gives excellent results. I have found that ascreen of this type of almost any desired size may be convenientlyconstructed by molding or by casting a suitable liquid plastic upon acorrugated matrix surface and allowing the plastic to set or harden.

A novel and convenient way of forming such a matrix surface isillustrated in Figs. 3, 4 and 5. For this purpose, I prepare a matrixsurface having a closely Wound spiral ridge thereon, it having beenfound that a closely wound spiral very closely approaches the desiredconcentric arrangement. The spiral matrix may be readily formed byemploying a core member l2 which has cut in the upper surface thereof aspiral groove i3, the cross section of which is so adjusted as toprovide a spirally wound ridge I4 having the same cross section as thedesired cross section of the groove l. The outermost turn of the ridgelli is preferably cut abruptly along the vertical plane passing throughthe center of the core Piece l2 to provide an end face l5 such as thatillustrated in Fig. 4. A corresponding and similar end i6 of a strip ofribbon material Il may be placed in contact With the end l5 and securedto the bodylof the core member l2 by means of a pin IB.

The pin l may be passed through a hole i9 provided in the ribbon Il andinto a corresponding hole 2li provided in the edge of the core memberl2. The ribbon material Il is then wound about the core member I2 so asto form a continuation of the spiral ridge lll. It will be observed thatthe second turn of the ribbon material Il covers the head of the pin I8toprevent it from coming out of the holes I9 and 20 in which it isreceived so that after the ribbon Il is wound one full turn it willremain secured to the core member I2. The ribbon ll is then wound aboutthe core member l2 in spiral fashion until a sheet having a diametergreater than the diagonal of the screen to be manufactured is thusformed.

The strip material I1 is preferably formed of a relatively soft metalhaving little, if any, resilience so that after it is wound in thespiral form it will not exert any great force in tending to straightenout and destroy such spiral shape. I have found that solder or a similaralloy composed chiefly of lead and tin .provides the desiredcharacteristics. The strip I1 may be provided with the required crosssectional con-l tour in any suitable manner such as by rolling orextruding or drawing through dies.

I have found it desirable to provide a means for securing adjacent turnsof the ribbon material I1 to each other to prevent relative movementbetween these turns. For this purpose I provide a recess 2| having arectangular cross section and extending longitudinally the full lengthof the ribbon I1. The recess 2I is adapted to receive a correspondingtongue or ridge 22 formed upon the opposite f-ace of the strip I1. Itwill be seen that as the strip is Wound spirally, adjacent turns areheld against relative movement by the tongue 22 entering the groove 2|.The entire matrix surface thus formed may be secured to a fiatsupporting surface or table 23 by providing upon the lower edge of theribbon a downwardly depending tongue portion 24 yand b'y providing inthis tongue portion at spaced intervals along the length of the strip I1openings 2-5.

A wire 26 or similar securing means may be passed through the openings25 and through suitable openings 21 provided in the table 23 so as tohold the entire matrix structure thereon.

After the matrix has been assembled and secured to its supportingsurface 23, the rough rectangular outline of the screen I may be definedby a temporary retaining wall 28 formed of Wax, clay, or similarmaterial. Within this retaining wall is then poured, sprayed orotherwise introduced a liquid plastic material 29 which is capable ofair-setting or hardening. After the plastic material 29 has set, theretaining walls 28 may be destroyed and the entire sheet may be removedfrom the matrix surface.

The sheet thus formed Will be roughly rectangular in shape and will haveformed upon its front face the previously mentioned spiral grooves 1.The sheet may then be trimmed to the desired size and suspended for usein any suitable manner. I have found that a convenient way of suspendingthe screen I thus formed within the frame 4 is to employ theaforementioned border portion 2. This b'order may be formed in themanner indicated in Fig. 2 by taking a strip of canvas 30 or othersimilar strong and porous material and folding it back upon itself withone edge of the strip disposed upon the upper surface of the screen bodyand the other edge disposed upon the lower surface of the screen body.Liquid plastic material may then be poured over the canvas 30 tocompletely impregnate the same and to act as an adhesive to secure thecanvas strip 3U to the screen I. The application of pressure at thepoint where the canvas overlies the screen body will result in thecanvas being imbedded therein so that there is formed a substantiallyintegral border portion 2 comprising the canvas strip 3U impregnatedwith overlying layers 3l of plastic material. The border portion 2 thenmay be perforated at intervals along its length to provide openings 32through which the lacings 3 may be passed.

It has been found lthat if the completed screen including the bodyportion I and the border portion 2 be slightly heated as by placing theassembly in the sun and then laced tightly within the frame 4 While in aheated condition, the subsequent lowering of the temperature will causethe screen to shrink and be pulled by the lacings 3 very tightly into atrue plane surface.

From the foregoing, it will appear that I have provided a novel matrixfor constructing a light transmitting screen which is very efficient andiS capable of many and diverse applications in the motion pictureindustry. Furthermore, I have provided a novel matrix upon which liquidplastic material may be cast to provide such a screen.

While I have shown and described the prderred embodiment of myinvention, I do not desire to be limited to any of the details ofconstruction shown or described herein, except as defined in theappended claims.

I claim:

l. In a matrix for forming a light transmitting screen of the typeemploying a grooved screen surface defining a plurality of facets forrefracting light received from a projector disposed on one side of saidscreen into a camera disposed on the other side thereof, the combinationof: a matrix body comprising a thin metal tape wound in spiral fashionabout itself, adjacent turns of said spiral lying in contact with eachother, said tape having a pre-formed edge portion of a cross sectioncongruent With said grooves for forming a matrix surface having aplurality of substantially concentric ridges thereon.

2. In a matrix for forming a light transmitting screen of the typeemploying a grooved screen surface defining a plurality of facets forrefracting light received from a projector disposed on one side of saidscreen into a camera disposed on the other side thereof, the combinationof: a matrix body comprising a thin metal tape Wound in spiral fashionabout itself, adjacent turns of said spiral lying in contact with eachother; an edge portion on said tape having a cross section congruentwith said grooves for forming a matrix surface having a plurality ofsubstantially concentric ridges thereon; a longitudinally extendingtongue formed on one side of said tape; and a longitudinally extendinggroove formed in the other side of said tape for receiving the tongue onan adjacent turn of said spiral to hold said adjacent turns againstrelative movement toward and away from said surface.

OTIS C. MARTIN.

